accounting: time-based balance snapshots

[Roasbeef]

During the recent call with several lnd users, the notion of being able to obtain a balance "snapshot" at any period of time came up. I think the use case here is going back and looking at the historical flow of funds within a channel.

FWIW, this is already available in lnd as is, since we store the entire "revocation" chain. However this isn't indexed by "time" and instead uses an auto-incrementing sequence number of each new channel state. The only notion of time for the revocation chain is the absolute expiation heights on any HTLCs possibly present on the commitment transaction.

Alternatively, this can be done by polling lnd periodically, or having faraday use the streaming balance RPCs to update its own local state.

[philipglazman]

+1 on time-based balance snapshots.

Related feature that would be useful is time-based balance snapshots for each Bitcoin address (on-chain wallet and channel funding txn outputs). I'm not sure how this fits into the Harmony Reporting Standard, so might be a different report entirely.